Method and device for fiber-optical measuring systems

ABSTRACT

The invention relates to a method for optical measuring systems, comprising a sensor element ( 6 ) connected to a measuring and control unit ( 10 ) via an optical connection ( 3 ), and being adapted for providing a signal defining a measurement of a physical parameter (p) influencing the sensor element ( 6 ), said method comprising generation of a measuring signal that is brought to come in towards the sensor element ( 6 ), and detection of the intensity of the measuring signal (B) in the measuring and control unit ( 10 ), after influencing the measuring signal in the sensor element ( 6 ). The invention is characterised by comprising partial reflection of the measuring signal at a point along the optical connection ( 3 ), at a predetermined distance from the sensor element ( 6 ), detection of the intensity of the signal (A), corresponding to said partially reflected measuring signal, and determination of a measurement of said parameter (p) based upon the intensity of the partially reflected signal (A) and the intensity of the measuring signal (B). The invention also relates to a device for carrying out said method. Through the invention, measurements with an optical pressure measuring system are allowed, which exhibit effective compensation for any existing sources of error.

TECHNICAL FIELD

The present invention relates to a method for measuring systems. Theinvention is especially intended for use with intensity-basedfibre-optical measuring systems for pressure measurements. The inventionalso relates to a device for carrying out such a method.

BACKGROUND ART

In connection with measuring physical parameters such as pressure andtemperature, it is previously known to utilise various sensor systems bywhich the optical intensity of a ray of light, conveyed through anoptical fibre and coming in towards a sensor element, is influenced dueto changes in the respective physical parameter. Such a system may forexample be used when measuring the blood pressure in the veins of thehuman body. Said system is based upon a transformation from pressure toa mechanical movement, which in turn is transformed into an opticalintensity, conveyed by an optical fibre, which is in turn transformedinto an electrical signal that is related to the measured pressure.

According to known art, such a fibre-optical measurement system maycomprise a pressure sensor, an optical fibre connected to said pressuresensor, and at least one light source and at least one light detectorlocated at the opposite end of the fibre, in order to provide thepressure sensor with light, and to detect the information-carrying lightsignal returning from the pressure sensor, respectively.

One problem occurring with previously known systems of the above kindrelates to the fact that the detected signal will be influenced byvarious interference factors in connection with the measuring system.For example, the signal may be influenced by any bending of the opticalfibre, and by temperature changes and ageing of the optical fibre or ofsaid light source. Also factors such as fibre couplings and fibreconnectors in the measuring system in question may influence theinformation-carrying signal (for example through influencing itsintensity in an unwanted manner) and thus also the final measuringresult.

As a result of the above problems there is a need for devices andmethods arranged for compensation of any existing sources of error andinterference in connection with optical measurements of for examplepressure.

There are several previously known measuring systems in which ameasuring signal is used together with a separate reference signal. Acertain measuring system category is based upon conveying light throughtwo different optical fibres, and is used for said purpose. One exampleof such a system is shown in the patent document U.S. Pat. No.5,657,405, which describes a fibre-optical measuring system used formeasuring of e.g. pressure. In this system, the interference occurringbetween two optical conduits through which two corresponding laser lightsignals are directed towards a membrane, is utilised. One of these lightsignals is hereby used as a reference signal.

Another previously known category of systems is based on generating andutilising light of two different wavelengths, whereby a reference signalmay be obtained. Systems of this kind are previously known from forexample the patent documents U.S. Pat. No. 5,280,173 and U.S. Pat. No.4,933,545.

One disadvantage with the systems according to the two categoriesmentioned above is that they are relatively complex in their structure.They further require a relatively large number of critical components inthe form of LED:s, optical fibres, etc.

DISCLOSURE OF INVENTION

A primary object of the present invention is to provide an improvedmeasuring system, with the aid of which unwanted influences from sourcesof error and interference in intensity-based fibre-optical measuringsystems can be minimised. This is achieved by means of a method and adevice in accordance with the present invention.

The invention is intended for use in optical measurement systemscomprising a sensor element connected to a measuring and control unitvia an optical connection, and that are adapted for providing a signalcorresponding to a measurement of a physical parameter acting upon thesensor element. The invention consists of a method comprising thegeneration of a measuring signal that is brought to come in towards thesensor element, and the detection of the intensity of the measuringsignal in the measuring and control unit, after influencing themeasuring signal in the sensor element. The invention is characterisedby comprising partial reflection of the measuring signal at a pointalong the optical connection, at a predetermined distance from thesensor element, detection of the intensity of the signal correspondingto said partially reflected measuring signal, and determination of ameasurement of said parameter based on the intensity of the partiallyreflected signal and the intensity of the measuring signal.

Through the invention a substantial advantage is achieved, as it can beutilised in a simple and effective manner for compensation of sources oferror and interference by intensity-based optical measurements of e.g.pressure.

It is a further object of the invention to provide a method for anoptical measuring system, wherein a signal is brought to come in towardsa sensor element, and wherein a measurement of the length of an opticalconnection between said sensor element and a measuring and control unitcan be determined in a simple and efficient manner. This measurement canin turn be used to obtain improved measurements.

Said method is based especially upon a determination of a measurement ofthe length of said optical connection, based on a measured period oftime passing from the generation of said signal and up to the detectionof said signal. With such a method, the length determination may be usedfor identification of which sensor element that is currently beingconnected to the subject measuring and control unit. Hereby, the lengthof the optical connection is chosen so as to correspond to a specifictype of sensor element.

Advantageous embodiments of the invention are defined by the subsequentdependent claims.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained in more detail below, with reference toa preferred embodiment and to the enclosed drawings, in which:

FIG. 1 shows, schematically, a measuring system according to the presentinvention;

FIG. 1 a shows an enlarged view of a sensor element suitable for use inconnection with the invention; and

FIG. 2 shows a graph illustrating how light signals are detectedaccording to the invention.

PREFERRED EMBODIMENTS

FIG. 1 shows, schematically and somewhat simplified, an intensity-basedfibre-optical measuring system 1 according to the present invention.According to a preferred embodiment, the measuring system is designedfor pressure measurements, but alternatively, the invention could beused e.g. for measuring temperature or acceleration.

To the measuring system 1 belongs a light source in the form of an LED 2functioning to emit a light signal of a predetermined wavelength λ₁. TheLED 2 is connected to an optical connection, preferably in the form ofan as such previously known optical fibre 3, by means of a first link 4and a fibre coupling 5. The optical fibre 3 is in turn connected to asensor element 6.

According to what is shown in detail by FIG. 1 a, which is an enlargedview of the sensor element 6, said element comprises a cavity 6 a, forexample obtainable (according to known art) through construction bymeans of molecular layers (primarily silicone, alternatively siliconedioxide or a combination of the two) and an etching procedure.Preferably, a bonding procedure is also utilised in assembling thevarious layers of the sensor element 6. The manufacture of such a sensorelement 6 is as such previously known, e.g. from the Patent DocumentPCT/SE93/00393. In this way, a membrane 6 b is also created within thesensor element 6, the deflection of which membrane will depend on thepressure p influencing the sensor element 6.

According to what will be described in detail below, the above lightsignal will be brought to come in towards the pressure sensor 6, i.e.towards its cavity 6 a. Hereby, the pressure p acting on the membrane 6b will modulate the light signal. When the membrane 6 b is influenced bya certain pressure p, the dimensions of the cavity 6 a, primarily itsdepth d, will change, entailing a modulation of the light signal throughoptical interference inside the cavity 6 a.

When designing the sensor element 6, the depth d of the cavity 6 a isselected to be a value of substantially the same magnitude as thewavelength λ₁ of the light signal. The sizing of the cavity 6 a isfurther made considering the required application area for the sensorelement 6, in the current case primarily the pressure range to which thesensor element 6 is to be adapted.

According to the invention, the light signal consists of a pulse ofrelatively short duration. In normal applications, using an opticalfibre 3 with a length of about 2–10 m, the pulse duration is in theorder of 10–50 ns. However, the invention is not so limited, but couldalso be realised with a pulse length deviating from this interval. Forexample, pulses of longer duration are used in those cases where verylong optical fibres (e.g. 100–200 m) are used.

The light pulse thus defines a measuring signal that is transmittedthrough the fibre 3 and fed into the sensor element 6. The light pulsewill be modulated in the manner described above by means of the cavity 6a and is thereby provided with information corresponding to the currentpressure p. The light signal modulated by the sensor element 6 is thentransmitted back through the fibre 3 and conveyed to a light-sensitivedetector 7, through said fibre coupling 5 and a further fibre link 8.The detector 7 is functioning to detect, in a known manner, theintensity of the reflected measuring signal.

For processing of the light signal detected by the detector 7, themeasuring system according to the invention also comprises an evaluationunit 9. The evaluation unit 9 thus defines, together with the LED 2, thelinks 4, 8, the coupling 5 and the detector 7, a measuring and controlunit 10, which in turn is connected to a presentation unit 11, e.g. inthe form of a display, by the aid of which a measurement of the currentpressure p can be visualised for a user.

The two links 4, 8 preferably consist of optical fibres of an as suchknown kind, the fibre coupling 5 thereby comprising an as such knownfibre junction device designed so as to transfer the two fibre links 4,8 into the fibre 3 leading to the sensor element 6.

It is a basic principle behind the invention that a semi-reflectingdevice 12 is provided along the optical fibre 3, at a predetermineddistance from the sensor element 6. This device 12, according to theembodiment, consists of a so-called ferrule, i.e. a separate, tube-likeunit for interconnection of optical fibres, arranged in such a mannerthat the light pulse emitted from the LED 2 will be partially reflectedback to the detector 7, i.e. without having run up to and beinginfluenced by the sensor element 6. The optical connection 3, accordingto the embodiment, is thus in practice comprised of a first opticalconductor 3 a that is coupled to a second optical conductor 3 b via saidferrule 12. Between the two optical conductors 3 a, 3 b, a small air gapis hereby provided by means of the ferrule, at which gap said partialreflection will occur.

The invention is not limited to the reflecting device 12 describedabove. Alternatively, other forms of mirrors, or reflecting coatings andsurfaces, may be used to provide a partially reflecting device creatingthe described light reflection.

Out of the light pulse emitted by the LED 2, two returning light pulsesare thus created, i.e. a measuring signal that reaches the sensorelement 6 and is then returned, and a reference signal that is returneddirectly at the reflecting device 12.

The returning light signals will run, via the fibre coupling 5, into thesecond fibre link 8 and to the detector 7. The detector 7 will herebydetect the intensity of the measuring signal and the reference signal,respectively. Because the reflecting device 12 is arranged at apredetermined distance from the sensor element 6, the reference signalwill reach the light detector 7 a short time period before the measuringsignal, reflected at the sensor element 6, will reach the light detector7. The time period elapsing between the detection of the two signalswill hereby depend on the position along the optical fibre 3 at whichthe reflecting device 12 is arranged, i.e. said period of time willdepend on the distance between the reflecting device 12 and the sensorelement 6.

With reference to FIG. 2, there is shown, schematically, how two pulsesgenerated in the above manner are detected by means of the detector 7.FIG. 2 thus illustrates the intensity I of the detected pulses, as afunction of time t. From the figure it can be gathered that a firstpulse A, resulting from the above light signal being reflected againstthe reflecting device 12, reaches the detector 7, said detector 7 herebybeing adapted to determine a value of the intensity I_(A) of said pulseA. Furthermore, a second pulse B is coming in towards the detector 7 acertain period of time t₁ after the first pulse A having reached thedetector 7. The intensity 18 of the second pulse B is also detected bythe detector 7. The second pulse B hereby corresponds to the abovemeasuring signal, i.e. a light signal having been modulated in thesensor element 6 and thus containing information regarding the pressurep acting on the sensor element 6 (compare FIG. 1 a).

Furthermore, the evaluation unit 9 is adapted to calculate the quotientof the two intensity values of the respective pulses, that isI_(A)/I_(B). Through the invention, a measurement is thus obtained,where the measuring signal (i.e. the second pulse B) defines ameasurement of the pressure p, including the effects of any sources oferror, and where the reference signal (i.e. the first pulse A) onlycorresponds to the effects of any sources of error. Through calculatingsaid quotient, a measurement of the current pressure is obtained, wherefactors reflecting sources of error and interference have thus beencompensated for. This is of course an advantage, as it will lead to lessinterference-sensitive measurements. As examples of unwanted sources oferror, any bending of the optical fibre, temperature changes and ageingof the optical fibre or the LED 2, may be mentioned, as well as anychanges occurring in the fibre coupling 5.

In essence, it applies that the first pulse A defines a reference signalthat can be used to compensate for the effects of any interference inconnection with measurements with the measuring system according to theinvention.

In order to be able to separate the two pulses A and B during detectionin the detector 7, it is required that the period of time t₁ exceeds aminimum limit value. This limit value is depending on how high aresolution that can be achieved with the aid of the measuring andcontrol unit 10. For normal applications, this limit value t₁ is in theorder of 10 ns. For normal applications, with optical fibres of thelength 2–10 m, it is therefore suitable that the reflecting device 12 islocated at about half the distance between the measuring and controlunit 10 and the sensor element 6.

According to a variant of the invention (not shown in the figures), thelatter can be arranged so as to send one single pulse to two or morebranches, in turn comprising two or more optical fibres with acorresponding number of sensor elements. Along each one of the opticalfibres, a reflecting device of the above kind will then be provided. Bymeans of suitable location of the respective reflecting devices alongeach optical fibre, reference signals and measuring signals from eachbranch can be obtained and detected at predetermined intervals. In thisconnection, the length of each optical fibre and the location of eachindividual mirror device must be adapted in such a way that allmeasuring and reference signals can be individually separated. Thesesignals can then be detected and evaluated in a manner analogous withthe above description.

With the aim of providing an especially efficient pressure measurement,the invention could be used for detection of the periods of timeelapsing from the generation of a certain light pulse at the LED 2 untilit is detected in the detector 7. By means of measured values of suchperiods of time (and with knowledge of the propagation velocity of thelight pulses along the optical connection 3 in question) a measurementof the length of the optical connection between the measuring andcontrol unit 10 and the reflecting device 12, and between the measuringand control unit 10 and the sensor element 6, respectively, can becalculated. If the individual sensor element 6 is fitted to an opticalconnection given a predetermined, unique length, this type of detectioncan be utilised for carrying out an identity check of the individualsensor element. For example, a measured length of the optical connectionof 2 m could hereby be said to correspond to a first type of sensorelement, whereas a measured length of the optical connection of 3 mcould correspond to a second type of sensor element. In this way, theinvention could be used in such a manner that the measuring and controlunit 10, by detecting the length of a certain optical connection, firstidentifies what type of sensor element is currently connected.Subsequently, the measuring and control unit 10 may, during thecontinued measurements, utilise for example information regardingcalibration and other similar data, specifically relating to thecurrently connected sensor element. This type of information wouldhereby preferably be pre-stored in the measuring and control unit 10 andbe used for the individual sensor elements that a specific measuring andcontrol unit 10 is intended to be used with. Through introducing, forexample, data regarding the calibration of a specific sensor element tobe introduced into the measurements, the invention thus allows improvedmeasurements.

The invention is not limited to the embodiment described above, but maybe varied within the scope of the appended claims. For example, theprinciple behind the invention could be used also for systems notintended for pressure measurements.

Instead of a calculation of the quotient of two intensity values of twolight signals, i.e. I_(A)/I_(B) (according to the description above), acalculation of the difference (I_(A)−I_(B)) between said two valuescould be performed in the measuring and control unit. Also in this case,a compensation for any sources of error and interference is obtained.According to a further conceivable solution, the two light signalsI_(A), I_(B) could be comprised as input parameters in an appropriatelyformed function, by the aid of which a compensation for sources of errorwould be provided.

1. A method for optical measuring systems, comprising a sensor element connected to a measuring and control unit via one single optical fiber and being adapted for providing a signal corresponding to a measurement of a physical parameter influencing the sensor element, said method comprising the steps of: generation of a measuring signal that is brought to come in towards the sensor element, and detection of said measuring signal in the measuring and control unit by a single detector, after influencing the measuring signal in the sensor element, partial reflection of the measuring signal at a point along the one single optical fiber, located at a predetermined distance from the sensor element, detection of the intensity of the signal corresponding to said partially reflected measuring signal by said single detector, and determination of a measurement of said parameter based upon the intensity of the partially reflected signal and the intensity of the measuring signal.
 2. The method according to claim 1, characterized by comprising: determination of a value corresponding to the quotient of the intensity of said reflected signal and the intensity of said measuring signal, and determination of a measurement of said parameter based upon said quotient.
 3. The method according to claim 1, characterized by comprising: determination of a value corresponding to the difference between the intensity of said reflected signal and the intensity of said measuring signal, and determination of a measurement of said parameter based upon said difference.
 4. A method according to claim 1, characterized by said measuring signal being a light pulse.
 5. A method according to claim 1, characterized by the feeding of the measuring signal into the sensor element causing optical interference in a cavity of the sensor element.
 6. A method according to claim 1, characterized by being used for measuring pressure, said sensor element defining a membrane, acted upon by the pressure surrounding the sensor element.
 7. A method according to claim 1, characterized by being used for measuring the acceleration or the temperature of said sensor element.
 8. A device for optical measuring systems, comprising: a sensor element connected to a measuring and control unit via one single optical fiber and being adapted for providing a signal corresponding to a measurement of a physical parameter influencing the sensor element, a light source functioning to generate a measuring signal brought to come in towards the sensor element, a detector for detecting the intensity of the measuring signal in the measuring and control unit, after influencing the measuring signal in the sensor element, a semi-reflecting device for partial reflection of the measuring signal at a point along the one single optical fiber at a predetermined distance from the sensor element, said detector being arranged for detection of the intensity of the signal corresponding to said partially reflected measuring signal, and an evaluation unit for determining a measurement of said parameter, based upon the intensity of the partially reflected signal and the intensity of the measuring signal from said detector.
 9. The device according to claim 8, characterized by said sensor element comprising a cavity, shaped so as to create optical interference when feeding said measuring signal into the cavity.
 10. The device according to claim 9, characterized by said cavity being obtained through building up molecular silicone and/or silicone dioxide layers, and an etching procedure.
 11. The device according to claim 10, characterized by said cavity being obtained through utilizing a bonding procedure. 